Superplastic forming is emerging as an industrial process for making hard-to form aluminum sheet metal parts. The use of superplastic forming in commercial production of metallic sheet parts, especially aluminum sheets, should provide desirable improvements in both cost and efficiency. However, superplastic forming processes generally require the use of fine-grain sheet alloys, typically those having grain size of less than 10 microns. These fine-grain sheet alloys have traditionally been produced by imparting heavy cold plastic deformation to sheet metal through massive cold rolling reduction achieved in multiple rolling mill passes. A major concern for commercializing superplastic forming is that the process is inherently slow resulting in very long part forming times compared to the room temperature stamping process. High-rate superplastic forming has been demonstrated in many alloys, but requires the use of sheet metal having an ultra-fine grain microstructure, generally less than 1 to 2 microns. However, current industrial sheet metal processing done in traditional rolling mills has generally been unable to produce an ultra-fine microstructure.
Severe plastic deformation, through confined shear deformation, has been shown to produce ultra-fine grain size in aluminum alloys. Severe plastic deformation is usually achieved through procedures such as equal-channel angular pressing and high-pressure torsion. However, to date, neither of these procedures has been available for use in the processing of continuous metal strips or metal sheet stock.
A process known as continuous confined strip shearing has been proposed to address the disadvantages of equal-channel angular pressing. In this process, the friction forces from a feeding roll acting on an aluminum sheet or strip propel the sheet or strip along an upper die into a deformation zone having an angled channel. However, high friction forces acting from the upper die on the metal sheet and the deformation resistance in the deformation zone impede or stop the motion of the sheet. As a result, the sheet may slip and slide on the feeding roll, causing process instabilities and interruptions. Aluminum may also adhere to the surfaces that the sheet contacts, resulting in challenges for high-volume production processes.